Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a first image in a sequence for a display screen or a portion thereof with a transitional graphical user interface;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is front view of a third image thereof;

FIG. 4 is a front view of a fourth image thereof;

FIG. 5 is a front view of a fifth image thereof;

FIG. 6 is a front view of a sixth image thereof;

FIG. 7 is a front view of a seventh image thereof;

FIG. 8 is a front view of an eighth image thereof; and,

FIG. 9 is a front view of a ninth image thereof.

The appearance of the image transitions sequentially between the images shown in FIGS. 1-9. The process or period in which an image transitions to another forms no part of the claimed design.

The broken line outer perimeter of the Figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the Figures illustrating portions of the graphical user interface are included for the purpose of illustrating environmental structure and form no parts of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 